1. Field of the Invention
This invention generally relates to an electric discharge machining apparatus for controlling electrical discharges across the gap and, more particularly, to an apparatus for preventing abnormal discharges.
2. Description of Prior Art
In general, an electrical discharge between a workpiece and an electrode results in the removal of a small crater of material from the workpiece. Electric discharge machining may be performed by the application of a plurality of electrical discharges in a highly repetitive manner. The spacing between the electrode and the workpiece, called the gap, is typically on the order of a few microns, or tens of microns, and is typically filled with a dielectric machining liquid, such as water or kerosene. The pulses of current have a nearly constant peak current value and constant pulse width having an ON state and an OFF state for the predetermined machining condition.
In order to induce optimal electric discharge machining, the pulse-like discharges of current must be optimally controlled in accordance with the material and hardness of the workpiece and the discharge condition at the gap. The discharges across the gap must be optimally controlled in order to prevent abnormal discharges, such as a concentrated electric discharge, which could cause an undesirable large crater on the surface of the workpiece, or a continuous arc discharge, which could damage either the workpiece or the electrode by causing arc scars, from occurring at the gap. Control of the discharges is performed by having a controller supply pulsed 10 electrical power to the gap based on a detected discharge voltage at the gap or a discharge current at the gap.
Although the occurrences of abnormal discharges across the gap may be prevented by optimally controlling the discharges, such optimal control is dependent upon various factors, such as the flushing condition and the shape of the workpiece. Optimal control is also dependent upon use of an electrode jump function a function, in which the electrode moves up and away from the workpiece and then immediately returns down to a position close to the workpiece. Because of these and other varying factors, carbide formation may still occur at the gap even with so-called optimal control of the discharges. The carbide formation may hinder the progress of machining and arc scars may damage the workpiece or the electrode due to continuous arc discharges.
Thus, in order to prevent carbide formation, an operator typically must visually observe the machining and, if carbide formation is detected, the electric discharge machining would be halted. The detection of the carbide formation, however, is dependent upon the ability and skill of the operator to detect the carbide formations. Thus, some operators may not halt machining until the carbide has formed and the discharges have deteriorated into continuous arc discharges. At the this stage, the electrode and the workpiece may already be damaged by arc scars. On the other hand, other operators may halt the machining before any carbide has formed which would endure frequent stopping of the machining operation, thereby increasing the machining time and decreasing the machining efficiency,